1. Field of Invention
This invention relates to lithographic printing. In particular, this invention relates to a rewritable lithographic printing plate and systems and methods for rewriting the plate by controlling the reversible hydrophobic/hydrophilic properties of the surface of the plate.
2. Description of Related Art
Conventional lithographic printing plates are prepared with image-wise hydrophobic/hydrophilic areas. Water is then exposed to the hydrophobic/hydrophilic surfaces of the plate. The water avoids all of the hydrophobic areas, but clings to all of the hydrophilic areas. The surface of the plate is then exposed to an oil-based ink. Since the oil-based ink and the water are immiscible, the oil-based ink avoids the areas that are coated with water and adheres to the remaining areas. In other words, the oil only clings to the hydrophobic areas. The oil-based ink and water is then transferred to a blanket cylinder and then onto a recording medium, such as paper.
Conventional lithographic printing plates are generally prepared outside of printing presses. Thus, a plate must first be prepared using a dedicated printing plate preparation machine and then installed in a lithographic printing press. This preparation and installation wastes valuable time and must be performed for each image that is to be printed. This problem is compounded in color lithographic printing systems which require a different plate for each color of an image to be prepared and installed. Additionally, newly prepared plates cannot be installed without first removing and disposing of any plates that are already in the press and which are being replaced. The plates being replaced cannot be rewritten and, therefore, represent a significant waste of materials, energy and time.
The preparation time of conventional lithographic printing plates is also very lengthy. Each plate requires several minutes to prepare. Typically, blank lithographic printing plates have a hydrophobic surface which is conditioned to provide hydrophilic regions which are distributed on the surface in an image-wise manner. One example of a lithographic printing plate preparation process involves a blank lithographic printing plate having a surface that is coated with a hydrophobic photopolymer film. This film is exposed to light from a laser. The photopolymer reacts to the light and the light-exposed areas of the hydrophobic photopolymer film are removed by exposing the surface to a chemical solvent. This process is wasteful because the hydrophobic photopolymer film is not recoverable and the solvent requires special handling and control.
Another example of a conventional lithographic printing plate preparation method involves a blank lithographic printing plate having a surface coated with a hydrophilic silicone rubber film. The blank lithographic printing plate is also exposed to light from a laser in an image-wise manner. However, the laser removes the silicone rubber film and the chemical solvent exposing step is avoided.
Another conventional lithographic printing plate has a surface with an oleophobic silicone rubber film distributed in an image-wise manner. This type of plate may be used in a waterless lithographic printing process which has an advantage that the ink and the water do not have to be carefully balanced. The waterless lithographic printing plate has two different areas. A first area has an oleophobic silicone rubber film to which the ink will not bond and a second area which has had the oleophobic silicone rubber removed and which exposes an underlying substrate to which the ink will bond. The ink is then exposed to the surface of the plate and the ink only covers the areas where the silicone rubber has been removed. Subsequently, the ink is transferred to a blanket cylinder and then onto a recording medium.